Former 1000 Guineas favourite Certify and 14 other horses trained by Mahmood Al Zarooni have been suspended from running for six months by the British Horseracing Authority.
Last year's star juvenile was one of 11 horses that tested positive for anabolic steroids following a random BHA test at Al Zarooni's Newmarket stables on April 9.
Al Zarooni admitted administering the drugs in error and named a further four inmates that he had given banned substances to.
All the horses, which also include top stayer Opinion Poll, have had their ban backdated to April 9, the date of their first test.
Certify was one of seven horses to test positive for Ethylestranol while Opinion Poll was one of four whose blood was found to contain Stanozolol.
Godolphin owner Sheikh Mohammed has already locked down Al Zarooni's stables, saying he was "appalled and angered" by events and that testing for all the inmates at Moulton Paddocks was now planned.
Jamie Stier, the BHA's director of raceday operations and regulation, said: "The BHA has today informed Godolphin, the owners of the horses in question, that the 15 horses known to have been administered with either Ethylestranol or Stanozolol have been suspended from running in races for a period of six months, with effect from Tuesday, April 9, 2013.
"The length of suspension reflects the period beyond which the BHA is confident that the horses in question can have derived no performance related benefit from the administration of these prohibited substances."
Certify ended 2012 unbeaten in four runs, claiming the champion juvenile filly title after winning the Fillies' Mile at Newmarket on her final start of the campaign.
The suspensions will effectively rule all the 15 horses out for the entire campaign with just four and a half weeks left of the season on their return.